The Behavior of Gases

Compressibility Gases can expand to fill its container, unlike solids or liquids The reverse is also true: They are easily compressed, or squeezed into a smaller volume Compressibility is a measure of how much the volume matter decreases under pressure

Gas Particle Compression in an Air Bag This is the idea behind placing “air bags” in automobiles Particles of gas (in the airbag) compress more than particles of solid (in the steering wheel or dash board) The impact forces the gas particles closer together, because there is a lot of empty space between them

Variables that describe a Gas The four variables and their common units: 1. Pressure (P) in kilopascals 2. Volume (V) in Liters 3. Temperature (T) in Kelvin 4. Amount (n) in number of particles The amount of gas, volume, and temperature are factors that affect gas pressure. http://www.learnerstv.com/animation/animation.php?ani=123&cat=Chemistry

Amount of Gas When we add air to a bicycle tire, we are adding gas molecules. More gas particles means more collisions thus, the pressure increases This is why the tire feels harder after pumping it up Relationship: # gas particles __, Pressure ___ Before pumping After pumping

Pressure Gradient Gas particles naturally move from areas of high pressure  low pressure Example – can of whipped cream, hair spray, spray paint Stored with air under high pressure When you press the nozzle, the gas is free to move – and it moves out of the can because atmospheric pressure is lower than the pressure inside the can.

Pressure Gradient Why do the instructions on an aerosol can say to hold the can upright while spraying? Compressed air is less dense than product, so air moves up… Air enters the tube instead of product

Volume of Gas In a smaller container, the molecules have less room to move. The particles hit the sides of the container more often. As volume decreases, pressure increases

Temperature of Gas Raising the temperature of a gas increases the pressure, if the volume is held constant. Remember, temperature is a measure of kinetic energy Gases at higher temp. have more kinetic energy: The molecules hit the walls harder, and more frequently What would happen if you threw an aerosol can into a fire? What happens to a helium filled balloon on a cold day? When should your car tire pressure be checked? Heat from fire adds energy, as energy , temp.  and pressure . When can can’t withstand pressure, it explodes.

The Gas Laws V, P T , V T, P Boyle’s Law – Pressure and Volume If temperature is constant, as the volume of a gas is decreased, the pressure increases. Charles’s Law – Temperature and Volume If pressure is constant, as temperature of an enclosed gas increases, the volume increases Gay-Lussac’s Law – Pressure and Temperature If volume is constant, as temperature of an enclosed gas increases, the pressure increases V, P T , V T, P

The Combined Gas Law The combined gas law expresses the relationship between pressure, volume and temperature of a fixed amount of gas. All temperature measurements MUST be in Kelvin “PLEASE VOMIT OVER TOILET”

Ideal Gases An ideal gas is a gas that obeys the gas laws under ALL conditions of temperature and pressure. It does this because: The particles themselves have no volume There are no attractive forces between the particles However… An ideal gas does not really exist!

Ideal Gases don’t exist, because: Gas particles do take up space - In other words, they do have volume There are attractive forces between particles - otherwise there would be no liquids formed

However, Real Gases behave like Ideal Gases ... When the particles are very far apart and do not take up as big a % of the space We can ignore the particle volume. This happens at low pressure

Real Gases behave like Ideal Gases… When particles are moving fast This is at high temperature Collisions are harder and faster. Particles are not next to each other very long. Attractive forces cannot play a role.

Why we work with Ideal Gases Although ideal gases do not really exist, real gases can behave like ideal gases at low pressure and high temperature, and so… If we ASSUME a gas is ideal: it is much easier to predict how it will behave, the math is much easier and in reality it is a very close approximation for how the real gas will behave.

Dalton’s Law of Partial Pressures Dalton’s Law of partial pressure states that at constant volume and temperature, the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the component gases

If the first three containers are all put into the 4th, we can find the pressure in the 4th container by adding up the pressure in the first 3: = 6 atm 2 atm + 1 atm + 3 atm 1 2 4 3

Dalton’s Law of Partial Pressures For a mixture of gases in a container, PTotal = P1 + P2 + P3 + . . . P1 represents the partial pressure of gas in container 1 (or the contribution that the gas in container 1 made to the total pressure)